A great many types of actuator systems are known from diverse technical fields. Actuator systems range from relatively large, slow and imprecise devices, to relatively small, rapidly actuating and highly precise systems such as the type known from fuel injectors and other fluid control systems. In many instances, it is desirable to bias an actuator toward a particular state, such as a predefined position. In the context of fuel injectors, biasing springs are commonly used to bias an actuator or related components toward one of multiple possible positions. For example, return springs are often used in connection with control valves and associated electrical actuators to bias the electrical actuator toward a position corresponding with a particular fluid state of a fuel injector such as a rest state when the fuel injector is not injecting fuel into an engine cylinder. With the ever increasing focus on precision, reliability and predictability in actuator systems, notably fuel injector actuator systems, the behavior of return springs and other biasing devices is increasingly important.
Another common application of biasing devices is providing a preload on certain types of actuators such as piezoelectric actuators. Like other common actuator systems used in fuel injectors, piezoelectric actuators may be activated or deactivated to control the position of a control valve or the like. To enable operation and to ensure accurate, reliable and predictable performance, many piezoelectric actuators are preloaded via a compressive force. Piezoelectric actuators tend to be somewhat sensitive to variations in preload. Moreover, as engine conditions such as temperature change, the dimensions and spacing of piezoelectric actuator components can change. The magnitude of preloading force on piezoelectric actuators used in fuel injectors may be tightly specified for these reasons. As alluded to above, the behavior of return springs and other biasing devices may be important in ensuring desired operation of an actuator. It has been observed that variability in spring construction and design can negatively impact the ability of a biasing spring to perform as desired in certain actuator systems. In the case of piezoelectric actuators, the biasing spring may be used not only as a preloading device, but also as a return spring to return the actuator to a state it normally occupies when de-energized, compounding these issues.
U.S. Pat. No. 7,025,292 to Mohr sets forth one example of a fuel injector having a valve biased via a biasing spring. The spring used in Mohr has the form of a cylindrical sleeve with cut-out regions to impart a spring action to the sleeve. Mohr may have certain applications, but would be considered inferior in others, particularly if applied in certain piezoelectric actuator systems.